Low grade heat, which is typically considered less than 100 degrees, represents a significant waste energy stream in industrial processes, power generation and transport applications. Recovery and re-use of such waste streams is desirable. An example of a technology which has been proposed for this purpose is a Thermoelectric Generator (TEG). Unfortunately, TEG's are relatively expensive. is Another largely experimental approach that has been proposed to recover such energy is the use of Shape Memory Alloys.
A shape-memory alloy (SMA) is an alloy that “remembers” its original, cold-forged shape which once deformed returns to its pre-deformed shape upon heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems.
The three main types of shape-memory alloys are the copper-zinc-aluminium-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but SMAS can also be created, for example, by alloying zinc, copper, gold and iron.
The memory of such materials has been employed or proposed since the early 1970's for use in heat recovery processes and in particular by constructing SMA engines which recover energy from heat as motion.
In a first type, referred to as a crank engine, of which U.S. Pat. No. 4,683,72 is an example, convert the reciprocating linear motion of an SMA actuator into continuous rotary motion, by eccentrically connecting the actuator to the output shaft. The actuators are often trained to form extension springs. Some configurations require a flywheel to drive the crank through the mechanism's limit positions. A related type are Swash Plate Engines, which are similar to cranks except that their axis of rotation is roughly parallel to the direction of the applied force, instead of perpendicular as for cranks.
A second type are referred to as a pulley engines, an example of which is U.S. Pat. No. 4,010,612. In pulley engines, continuous belts of SMA wire are used as the driving mechanism. A pulley engine may be unsynchronized or synchronized. In unsynchronized engines, the pulleys are free to rotate independently of one another. The only link between different elements is rolling contact with the wire loops. In contrast, in synchronized engines, the pulleys are constrained such that they rotate in a fixed relationship. Synchronization is commonly used to ensure that two shafts turn at the same speed or keep the same relative orientation.
A third type of SMA engine may be referred to as field engines, an example of which is U.S. Pat. No. 4,027,479. In this category, the engines work against a force, such as a gravitational or magnetic field.
A fourth type of SMA engine is that of Reciprocating Engines of which U.S. Pat. No. 4,434,618 in an example. These reciprocating engines operate linearly, in a back-and-forth fashion, as opposed to cyclically.
A fifth type of SMA engine is that of Sequential Engines of which U.S. Pat. No. 4,938,026 is an example. Sequential engines move with small, powerful steps, which sum to substantial displacements. They work like an inchworm, extending the front part by a small step and then pulling the back part along. With the back part nearby, the front part can extend again.
A sixth type of SMA engine is shown in U.S. Pat. No. 5,150,770A, assigned to Contraves Italiana S.p.A., and discloses a spring operated recharge device. There are two problems with the Contraves device, namely it is difficult to recharge quickly in a reciprocating manner and secondly it is difficult to discharge the energy to a transmission system without losses occuring.
A seventh type of SMA engine is shown in US patent publication number US2007/261307A1, assigned to Breezway Australia Pty Limited, and discloses an energy recovery charge system for automated window system. Breezway discloses a SMA wire that is coupled to a piston which is used to pump fluid to a pressurised accumulator. The piston therefore moves in tandem with the SMA wire as it contracts and expands. By coupling the SMA wire to the piston in this manner, the SMA wire is in indirect communication with the energy accumulator via the pumped fluid which is inefficient and the Breezway system suffers from the same problems as Contraves.
Other patent publications in the art include U.S. Pat. Nos. 4,037,411; 4,030,298; WO2007/134088 and US2006/037315.
In addition one of the difficulties with each of these types of SMA engines has been that of the cycle period of the SMA material. SMA material is generally relatively slow to expand and contract (10's of RPM). It has been and remains difficult to achieve a worthwhile reciprocating frequency that might be usefully employed in an industrial application (100's to 1000's of RPM). This is not a trivial task and generally is complicated and involves significant parasitic power losses.
The present application is directed to solving at least one of the above mentioned problems.